David Cameron has held crisis talks at Downing Street after being told of allegations of a sensational love affair which has potentially significant political implications for him.

For legal reasons, The Mail on Sunday cannot disclose the identities of the people involved or any details of the relationship – even its duration – other than that they are middle-aged figures. The affair has now concluded.

But this newspaper can report that when aides told Mr Cameron the identities of the alleged lovers he was ‘stunned’, and, according to sources, ‘immediately realised the importance of the story’.

The Prime Minister and his aides also discussed the possible fallout should details of the affair become public – and how such disclosure could ‘blow out of the water’ any major political set pieces planned by No 10.

One senior source told this newspaper last night: ‘This revelation is dynamite. None of us could believe it when we first heard it. Then we just thought, “What a complete mess”.’

The source added that, apart from the political implications, the revelation had caused ‘great personal distress to innocent parties’.

It is understood that the Prime Minister was told of the relationship - which does not involve anyone serving in the Cabinet - within the past few weeks.

If details of the affair do emerge, it could place a further strain on Mr Cameron’s leadership, which is already being tested by backbench revolts over issues such as Europe and gay marriage and rumours of plots being hatched to overthrow him.

On Friday, he faced a further blow when one of his MPs, Patrick Mercer, resigned the party whip after being caught in a cash-for-questions ‘sting’ operation.

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If the affair is revealed, it is likely to cause as much public surprise as the disclosure of the relationship between John Major and Edwina Currie, which was kept secret for nearly two decades until 2002.

Like Mr Major, Mr Cameron is increasingly finding his time in Downing Street is beset by sex scandals.

Surprised: When aides told Mr Cameron the identities of the alleged lovers he was said to be 'stunned'. He is pictured with his wife Samantha during their holiday in Ibiza last week

Just weeks ago, the Commons was rocked by the news that Deputy Speaker Nigel Evans, a Tory MP, had been arrested following allegations that he raped one man and sexually assaulted another between 2009 and 2013 – accusations he has branded ‘completely false’.

A third person has also given a statement to Lancashire police, claiming to have been sexually assaulted by Mr Evans, and on Friday police said they were planning to interview a fourth alleged victim who claims he was intimately groped by Mr Evans in a Commons bar.

In April, Scotland Yard said it had launched a formal investigation into allegations of sexual misconduct by the Liberal Democrat peer Lord Rennard.

The Metropolitan Police set up a phoneline for alleged victims after a number of women made claims about inappropriate conduct by the peer, who was the party’s former chief election strategist.

Pressure: Mr Cameron is finding his time in Downing Street is beset by scandals. Deputy Speaker Nigel Evans, left, was recently arrested following allegations that he raped one man and sexually assaulted another . Patrick Mercer, right, was caught in a cash-for-questions ‘sting’ operation

Deputy Prime Minister Nick Clegg, who had initially denied knowing about the allegations, was later forced to concede that his office had heard ‘indirect and non-specific concerns’ as far back as 2008.

And, in a further case, Liberal Democrat MP Mike Hancock will tomorrow face a disciplinary hearing to determine whether he should lose the party whip over ‘very serious allegations’ of sexual assault.

Mr Clegg asked the party’s chief whip to convene the meeting after Hancock was served papers as part of a High Court civil action brought by a ‘vulnerable’ constituent.

When asked about the affair last night, a Downing Street source said: ‘This is not something we can talk about.’

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